Did you watch the Kurzgesagt video I included? https://www.youtube.com/watch?v=dGiQaabX3_o&t=368s

2.8 million years ago, the genus Homo (the first humans) emerged. So, like you said, it's not the species that primarily matters, it's the genus. So, we could possibly have mated with those creatures (were they still around) and produce fertile offspring. Homo Erectus survived for at least 2 million years. 10x longer than Homo Sapien Sapiens have existed. Yes, their populations were low. So much so, that it is estimated that all humans today have DNA from King Tut in Egypt, just because of how many children he had. At least 200 sons, and about 200 daughters. I'm not sure on mortality rates. But, if they had to have lots of children to sustain the populations of their tribes, than it's not farfetched that they would kill their child who might have been a threat to spreading disease throughout the tribe.